In general, an automobile including a working vehicle is provided thereon with an electric power steering device having an electric motor in a steering column or in a rack (Patent Document 1). The electric motor supplements a force required for a handle operation (steering) of a vehicle to lighten fatigue of an operator. This type of power steering devices according to the conventional technology is provided with a manual steering device that changes a direction of steering wheels in response to the handle operation of the operator. This steering device is provided with a torque sensor that is arranged in the middle of the steering column between a handle and a load portion of a steering mechanism to detect a steering torque by the operator, and an electric motor that generates an assistance torque for assisting in a steering torque that changes the direction of the steering wheels.
Here, the electric motor does not rotate as long as the handle is not operated by the operator, and the assistance torque by the electric motor is not outputted. In addition, the electric motor is controlled in a motor rotation such that as a detection torque by the torque sensor is the larger, the assistance torque can be made the larger. That is, the load torque applied on a steering system changes depending upon a road surface state such as slipperiness of a road surface. Therefore, in the conventional electric power steering device, the steering torque required for the operator to be applied on the handle varies. Accordingly, as the load torque becomes the larger, the steering torque (a steering feeling as a steering reaction of the operator) becomes the larger.
In this way, in vehicles such as the automobile running on a general road, a characteristic in which the steering torque changes depending upon the aforementioned road surface is sometimes used as a means for recognizing the road surface state. That is, the operator can recognize the state of the road from the operating feeling (steering feeling as the steering reaction) of the handle, which makes it possible to drive and steer with a resistant operating feeling during vehicle traveling.